Major redesign of the build system. Also allow icons to be installed optionally:

-DQUASSEL_ICONS=External
Will install Quassel's own icons to $PREFIX/share/apps/quassel rather than into the binary itself
-DOXYGEN_ICONS=External
Will not install our Oxygen subset into the binary and rely on them being installed externally instead

Build system is still work in progress, and I have likely b0rked building on Win and Mac.